Exosomes are small vesicles 40-100 nm in diameter, that are secreted by a number of different cell types for communicating with other cells via the proteins and ribonucleic acids they carry. Depending on their cellular origin, exosomes carry a uniquely distinct profile of proteins, which can trigger signalling pathways in other cells and/or transfer exosomal products into other cells by exosomal fusion with cellular plasma membranes. The protein composition of exosomes is distinct from that of other organelles, including early endosomes and plasma membranes, more closely resembling that of late endosomes or multivesicular bodies, (MVBs).
Exosome release has been demonstrated from different cell types in varied physiological contexts. For example, it has been demonstrated that B lymphocytes release exosomes carrying class II major histocompatibility complex molecules, which play a role in antigenic presentation (Raposo et al., J. Exp. Med., 183:1161, 1996). Similarly, it has been demonstrated that dendritic cells produce exosomes (i.e., dexosomes, Dex), which play a role in immune response mediation, particularly in cytotoxic T lymphocyte stimulation (Zitvogel et al., Nature Medicine, 4:594, 1998). Further, it has also been demonstrated that tumor cells secrete specific exosomes (i.e., texosomes, Tex) carrying tumor antigens in a regulated manner, which can present these antigens to antigen presenting cells. The application of exosomes for use as cancer vaccines has been reviewed by Tan et al., Int. J. Nanomed., 5:889-900, 2010.
Nef is a protein expressed by primate lentiviruses, such as HIV and SIV. Nef is known to be secreted in association with exosomes and has been also shown to be present on the surface of HIV-infected cells. Nef-expressing cells have a dramatically altered subcellular morphology and have been shown to induce the intracellular accumulation of multivesicular bodies and the extracellular accumulation of exosomes. Exosomes have been postulated to play a role in the production of HIV-1 virions. The so called “Trojan Exosome” hypothesis suggests that HIV-1 particles can “piggyback” on the process of exosome biogenesis to provide a means of transfer of infectious particles from one cell to another (Izquierdo-Useros et al., PLoS pathogens, 6(3):1-9, 2010). Although some of the aspects of this theory have been questioned, the research has established a precedent for HIV-1 proteins being carried out of the cell and from one cell to another via the exosome network.
There is great interest in exploiting the properties of exosomes for diagnostic, vaccination, and therapeutic applications, including new and effective methods for preparing recombinant proteins at an industrial scale, for vaccine preparation, and for immunotherapy. The present invention provides compositions and methods for exosomal expression of recombinant proteins.